HFSS Tutorial
1

Microstrip Patch Antenna

The microstrip patch antenna is a popular printed resonant
antenna for narrow-band microwave wireless links that require semi-hemispherical
coverage. Due to its planar configuration and ease of integration with
microstrip technology, the microstrip patch antenna has been heavily studied and
is often used as elements for an array. In this tutorial, a 2.4 GHz microstrip
patch antenna fed by a microstrip line on a 2.2 permittivity substrate is
studied. The following topics are covered:

Model Setup

First the model of the microstrip patch antenna has to be
drawn in HFSS. It consists of rectangular substrate and the metal trace layer as
shown in Fig. 1. Note that a quarter-wave length transformer was used to match
the patch to a 50 Ohm feed line. The dimensions of antenna can be found in the
HFSS simulation file.

Figure 1. Patch antenna layout showing substrate and patch trace.

Waveport Setup

In order to excite the structure an excitation source has
to be chosen. For this simulation a waveport will be used. The waveport will
excite the first mode of the microstrip line (quasi-TEM) and then HFSS will use
this field to excite the entire structure. In order to get an accurate result,
the waveport has to be defined properly; if it is too small the field will be
truncated (characteristic impedance will be incorrectly calculated) and if it is
too large a waveguide mode may appear. Please refer to the tutorial on defining
a waveport for further information. Since the substrate height is 1.57 mm and
the feed line width is 4.84 mm, the waveport size chosen is 5 mm high by 50 mm
wide. After the waveport rectangle is drawn, the WAVEPORT excitation was
assigned to it. In the Analysis section of this tutorial, it will be shown that
this waveport size accurately models the desired microstrip mode.

Airbox and Boundary Conditions

An airbox has to be defined in to model open space so that the radiation from
the structure is absorbed and not reflected back. The airbox should be a
quarter-wavelength long of the frequency of interest in the direction of the
radiated field. In the directions where the radiation is minimal, this
quarter-wavelength condition does not have to be met and an air “space” may not
even have to be defined. Since the radiation of a patch antenna is concentrated
at broadside, a rectangular box enclosing the structure is only needed; the
height of the airbox is 31.25 mm (quarter-wave at 2.4 GHz). The antenna with
airbox and waveport setup is shown in Fig. 2.

Figure 2. Patch antenna layout showing airbox and waveport.

Next, the 4 side faces and the top face of the airbox were
selected (Press F to select faces and O to select objects) and RADIATION
boundary was applied. Then the bottom face and the patch antenna trace were
selected and a FINITE CONDUCTIVITY boundary using Copper was assigned.

Meshing

Manually meshing should be performed on the airbox to get accurate results
for the antenna properties such as efficiency, directivity, and radiation
pattern. One should seed the airbox lambda/10. For this structure the initial
mesh length for the airbox was set to 12.5 mm (lambda/10 at 2.4 GHz). Fig. 3
shows the mesh property window.

Figure 3. Mesh setup window.

Analysis/Sweep Setup

A Solution Setup is added to the analysis of the simulation with the
following:

In addition, in the Options tab of the Solution Setup, the Minimum Converged
Passes was changed to 3. Since a Fast Sweep from 1 GHz to 5 GHz (401 points)
will be chosen, the solution frequency should line within the frequency sweep
range and around the passband (i.e, around 2.4 GHz). In addition, the field data
is saved for each frequency point in the sweep; field data needs to be saved in
order to do any field post-processing.

Before running the simulation, an additional Solution Setup was added with Solve
Ports Only to verify the waveport setup. This Port Only Setup was run and the
resulting port mode is shown in Fig. 4; a characteristic impedance of 50.7 Ohms
was obtained.

Figure 4. Port mode showing electric-field.

Plotting Results

The resulting return loss of the structure is shown in Fig. 5.

Figure 5. Return loss of antenna from 1 GHz to 5 GHz.

From Fig. 5, the fundamental resonance of the antenna occurs
at 2.36 GHz with a return loss of -29.43 dB. Next, the top face of the substrate
was selected and the Electric Field Vector was plotted for 2.36 GHz. The field
plot is shown in Fig. 6 and shows the expected half-wavelength field
distribution.

Figure 6. E-field distribution on antenna at 2.36 GHz.

To plot the far-field patterns of the antenna, a far-field
setup has to be created. Two will created; one for the E- and H-Plane
two-dimensional patterns and another for the three-dimensional pattern. To
create each far-field setup go to HFSS>Radiation>Insert Far-Field Setup>Infinite
Sphere. For the two-dimensional pattern, the default values have to be changed;
Phi should start at 0 deg and stop at 90 deg with a 90 deg step size. For the
three-dimensional pattern, the default values can be used. Fig. 7 shows the
two-dimensional patterns and Fig. 8 shows the three-dimensional patterns. To
obtain the radiation efficiency, peak gain, etc. go to HFSS>Radiation>Compute
Antenna/Max Param and choose 2.36 GHz as the frequency of interest.